This invention relates to the inspection and testing of valves for leakage, particularly valve packing leakage along the stem and/or leakage along or around that portion of the valve structure commonly known as the back seat, and to the removal of defective or worn valve packings from the valve.
A typical valve structure to which the present invention is applicable includes a valve body defining a flow passage therethrough and a movable flow control member for opening and closing said flow passage. A valve stem is connected to said member and is movable between first and second positions to effect movement of the flow control member through or between open and closed positions respectively. The valve has a stuffing box through which said stem extends and a packing in said stuffing box for engaging the stem. Gland means are provided for compressing the packing to inhibit flow of pressurized fluid outwardly along said stem from said flow passage. The stem and valve body further have co-operable sealing surfaces engageable with each other when the stem has been moved to said first position to prevent or inhibit flow of pressurized fluid outwardly of the valve and along said stuffing box between the packing and stem disposed therein. The above noted co-operable sealing surfaces are typically referred to as the "back seat" surfaces.
In the maintenance of valves, valve packing leakage along the stem due to excessive leakage past the back-seat and/or packing material requires that the packing set be periodically replaced with new material. Due to safety considerations and the complex procedure required for system and valve isolation, such as in nuclear plants, back-seat testing is not frequently done and the leakage is repaired by replacing the valve packing. In many oases the valve is dismantled to visually check the back seat only to find that it is adequate thus giving rise to great wastage of time and materials.
The present practice of valve packing testing and replacement is very labour-intensive, using hand tools such as cork screw devices, special packing picks, and/or water jets to cut or fragment the packing for withdrawal from the stuffing box. Further removal of the lantern or junk rings is very labour-intensive. These existing inspection, testing and packing removal methods are of limited value because:
1. There is a potential risk to operators arising from uncontrolled leakage of dangerous fluids during a back-seat testing program.
2. There is the potential risk of over-pressurizing the piping system when using compressible fluids to check for back-seat integrity causing packing blow-out on related valves.
3. Small hand tools are ineffective in removing hard, highly consolidated packing because penetration is difficult (e.g. asbestos-based packing under conditions of high pressure and temperature).
4. The use of small hand tools is ineffective in removing soft, stratified packing that come out in flakes or small chunks (e.g. graphite-based packings).
5. The metal scraping type tools can damage accurately machined stems or gland surfaces.
6. High maintenance costs are associated with a valve "tear-down" especially when such "tear down" is unnecessary as when visual inspection of the back-seat surfaces is deemed necessary only to find later that the back-seat is adequate and that the "tear down" time has been wasted. The planning and procurement of replacement materials, as well as high man-rem dosage accumulated in nuclear operations are further factors to consider.
7. Potential health risks to workers can arise due to airborne contaminants such as asbestos fibers.
8. Potential safety risks to the operators resulting from using high-pressure water jets to cut valve packing include the risk of splashing and the spread of contaminants.
9. The use of compressible fluid, such as bottled gas, as is sometimes used for jacking packings out of a stuffing box, is dangerous because of the risk of explosive packing release and the spread of contaminants.
10. Existing devices present great difficulty in withdrawing lantern and junk rings which can not be penetrated by hard tools or water jets. Other packing removal devices employing elongated cutters and pulverizers have similar disadvantages with limited application and long set-up times.